Polymer compositions, polymer films and methods and precursors for forming same

ABSTRACT

Stable, high performance polymer compositions including polybenzimidazole (PBI) and a melamine-formaldehyde polymer, such as methylated, poly(melamine-co-formaldehyde), for forming structures such as films, fibers and bulky structures. The polymer compositions may be formed by combining polybenzimidazole with the melamine-formaldehyde polymer to form a precursor. The polybenzimidazole may be reacted and/or intertwined with the melamine-formaldehyde polymer to form the polymer composition. For example, a stable, free-standing film having a thickness of, for example, between about 5 μm and about 30 μm may be formed from the polymer composition. Such films may be used as gas separation membranes and may be submerged into water for extended periods without crazing and cracking. The polymer composition may also be used as a coating on substrates, such as metal and ceramics, or may be used for spinning fibers. Precursors for forming such polymer compositions are also disclosed.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract NumberDE-AC07-05ID14517 awarded by the United States Department of Energy. Thegovernment has certain rights in the invention.

TECHNICAL FIELD

Embodiments of the present disclosure relate to polymer compositionsincluding polybenzimidazole, structures formed from such polymercompositions and precursors and methods for forming such polymercompositions. More particularly, the disclosure relates to N-substitutedpolybenzimidazole that is cross-linked with methylated,poly(melamine-co-formaldehyde) to form the polymer compositions, such asan interpenetrating polymer network.

BACKGROUND

Polybenzimidazole (PBI), or poly-2,2′(m-phenylene)-5,5′-bibenzimidazole,is a polymer that is resistant to strong acids, bases, and hightemperatures (e.g., up to 500° C.). It has a heat resistance temperatureabove 430° C. PBI also exhibits excellent, mechanical strength, wearresistance and chemical resistance properties. Therefore, PBI can beused for applications that operate under extremely high temperature,mechanical loads and chemical corrosive environments.

PBI has been used over the past years to form membranes, electricallyconductive materials, fire resistant materials, ultrafilters, and othertypes of separatory media. For example, fibers may be formed from PBI(i.e., PBI fibers) and woven into fabrics that are used in hightemperature fire resistant suits, which are able to withstandtemperatures of up to 600° C. Thin films or flat sheets of PBI aretypically swollen with other solvents, such as phosphoric acid, forstability and electrical conductivity. Solid forms (e.g., rods andblocks) of PBI may also be formed by heat compression molding of apowdered PBI resin.

However, the poor film-forming characteristics of PBI have prevented useof PBI in film or coating applications. PBI has very poor solubility inconventional organic solvents, but may be dissolved in highly polar,aprotic organic solvents, such as dimethyl sulfoxide (DMSO),N,N-dimethylacetamide (DMAc), N,N-dimethylforamide (DMF), orN-methylpyrrolidinone (NMP), to form a PBI solution. PBI's molecularstructure is dependent upon hydrogen bonding by its imidazole groups.Disrupting the hydrogen bonding causes crazing and cracking that isespecially pronounced when forming thin, flat articles, such as films.Therefore, additives (i.e., acidic compounds or lithium salts) may beintroduced to the PBI solution to stabilize the hydrogen bonding in thepolymer matrix.

Most commonly, PBI is stabilized using acidic compounds, such assulfuric acid, phosphoric acid and organic derivatives thereof. Suchacidic compounds do not result in substitution of PBI with sulfate orphosphate, but instead result in protonation of a polymer backbone ofPBI. While not wishing to be bound by any particular theory, it ispostulated that the acidic compounds penetrate the PBI matrix, breakingup the crystal ordering. Thus, combining PBI with the acidic compoundstends to “plasticize” PBI, resulting in formation of PBI gels, which areoften referred to as “dopes.” The PBI gels or dopes are used inconstructing PBI hydrogen fuel cells (or similar devices) whereintegration of water into the matrix is important for proton transport.However, a PBI gel or dope is not useful for solution casting of dry,free-standing films, especially when the resulting films will be exposedto moisture in their applications. In particular, the additives in thePBI solutions are known to cause deleterious effects, such as swelling,in PBI films when exposed to water or moisture. When the additives arenot used, solution-cast PBI films tend to stress crack and fissure,especially upon exposure to increased temperatures (e.g., temperaturesgreater than 150° C.) due to loss of the high boiling solvent. Suchdifficulties have prevented the formation of free-standing films of PBI.

As an alternative to forming PBI solutions including acidic compounds,PBI may be modified at the molecular level. PBI includes imidazolegroups with reactive nitrogen atoms (i.e., imidazole nitrogen atoms)that may be used for molecular substitution (i.e., grafting) or forforming new PBI polymers using aldehyde and amine precursors (monomers)prior to polymerization. Altering monomers of PBI before polymerizationis difficult, and the molecular morphology of the resulting polymer maybe considerably different from that of homogeneous PBI. SyntheticN-substitution of PBI after polymerization has been performed withvarying success. However, additional synthetic acts and work-upprocedures are required to form N-substituted PBI compounds.Furthermore, the hydrogen bonding within the polymer matrix is disrupteddue to the N-substitution at the imidazole nitrogen atoms. The additivesare often used to stabilize the polymer matrix of N-substituted PBIcompounds, thus, complicating formation of stable free-standing filmsfrom PBI.

Polymer compositions that include PBI crosslinked and/or blended withother polymers have also been used for PBI film formation. Althoughblending other polymers with PBI may provide some stabilization of thepolymer matrix, the majority of such polymer compositions are gels ordopes that are used for fibers or fuel cells. For example, a blend ofPBI and ULTEM® 1000 polyetherimide in DMAc has been used to form fibers.However, thin films formed from such a blend phase separate, turn opaqueand/or fracture without the addition of the previously describedadditives, especially in the presence of moisture.

BRIEF SUMMARY

In some embodiments, the present disclosure includes a polymercomposition. The polymer composition may include at least onepolybenzimidazole segment and at least one melamine-formaldehyde polymersegment at least partially intertwined with the at least onepolybenzimidazole segment. For example, the polymer composition mayinclude and an interpenetrating polymer network (IPN) of the at leastone polybenzimidazole segment and the at least one polybenzimidazolesegment.

In further embodiments, the present disclosure includes a precursor of apolymer composition. The precursor comprises a solution ofpolybenzimidazole and methylated, poly(melamine-co-formaldehyde).

In yet further embodiments, the present disclosure includes a method forforming a polymer composition. The method comprises combiningmethylated, poly(melamine-co-formaldehyde) and polybenzimidazole to forma mixture and heating the mixture to form a polymer composition.

BRIEF DESCRIPTION OF THE DRAWING

While the specification concludes with claims particularly pointing outand distinctly claiming that which is regarded as the present invention,advantages of this invention may be more readily ascertained from thefollowing detailed description when read in conjunction with theaccompanying drawing in which:

FIG. 1 is a Fourier transform infrared (FT-IR) spectrum showing acomparison of polybenzimidazole and polymer compositions formedaccording to embodiments of the present disclosure.

DETAILED DESCRIPTION

Polymer compositions including polybenzimidazole (PBI) and amelamine-formaldehyde polymer are disclosed. The polybenzimidazoleincludes a plurality of imidazole functional groups, each including twonitrogen atoms to which the melamine-formaldehyde polymer may be bonded.Thus, polybenzimidazole may be substituted with, or cross-linked by, themelamine-formaldehyde polymer. In addition, segments of thepolybenzimidazole and the melamine-formaldehyde polymer may be at leastpartially interlaced with one other. The polymer compositions may beused to form films, coatings, bulk materials (e.g., support rods) orfibers. The polymer compositions may be formulated to exhibit improvedstability in comparison to conventional PBI compositions (e.g., PBIgels), which enables formation of free-standing films and coatings. Forinstance, the polymer compositions exhibit substantially increasedresilience in comparison to conventional PBI compositions and may, thus,flex or bend as easily. The polymer compositions of the presentdisclosure may be substantially free of additives, such as acidiccompounds or lithium salts, conventionally used to stabilize PBIcompositions. The films and coatings formed from the polymercompositions, therefore, are not susceptible to swelling in the presenceof water and may be submerged under water for long periods of timewithout degrading. A precursor of the polymer composition and a methodfor forming the polymer composition are also disclosed.

As used herein, the term “melamine-formaldehyde” means and includespolymers formed by a condensation reaction of melamine(2,4,6-triamino-s-triazene) with formaldehyde (CH₂O). Examples of suchmelamine-formaldehyde polymers include, but are not limited to,methylated, poly(melamine-co-formaldehyde),poly(melamine-co-formaldehyde) butylated, poly(melamine-co-formaldehyde)isobutylated, and poly(melamine-co-formaldehyde) methylated/butylated(55/45).

As used herein, the term “free-standing” means and includes an articlethat is physically stable and retains its shape without a supportstructure, such as a substrate.

As used herein, the terms “interpenetrating polymer network” and “IPN”mean and include a polymer that includes two or more polymer segmentsthat are at least partially interlaced on a molecular scale, but are notcovalently bonded to each other. The polymer segments may each include acrosslinked network of repeating structural units connected by covalentbonds. The polymer segments may be interlaced such that separation ofthe repeating structural units from one another involves breaking thecovalent bonds.

As used herein, the term “imidazole” means and includes a 5-memberedcyclic structure (i.e., ring) having two nitrogen atoms and three carbonatoms in which the nitrogen atoms are at the 1- and 3-positions on thering, and the carbon atoms are at the 2-, 4-, and 5-positions on thering. The nitrogen atoms of the imidazole may be referred to herein as“imidazole nitrogen atoms.”

As used herein, the term “N-substituted polybenzimidazole” means andincludes polybenzimidazole having a chemical substituent attached to anitrogen atom thereof. For example, an N-substituted polybenzimidazolemay include the chemical substituent (e.g., methylated,poly(melamine-co-formaldehyde)) attached to at least one of the nitrogenatoms at the 1- and 3-positions of the imidazole ring. The term“N-substitution” means and includes the acts of bonding the chemicalsubstituent to at least one of the nitrogen atoms to form theN-substituted polybenzimidazole.

As used herein, the term “flowable” means and includes a sufficientlylow viscosity at room temperature that enables a material to changeshape or direction substantially uniformly in response to an externalforce (e.g., gravity or a weight of the material itself) such that thematerial readily flows out of a container or across a surface at roomtemperature.

As used herein, the term “permeability” means and includes a rate atwhich a gas passes through a membrane or film after the gas has come toequilibrium therein.

As used herein, the term “time lag” means and includes an amount of timefor a gas to permeate from a feed side of a membrane or film to apermeate side of a membrane or film opposite the feed side. The time lagmay be used to calculate a gas permeability of the membrane or film.

As used herein, the term “feed side” means and includes a side of amembrane or film that is exposed to a gas, for example, to determine gaspermeability.

As used herein, the term “permeate side” means and includes a side of amembrane or film through which a gas permeates from the feed side afterthe gas has reached equilibrium in the membrane or film.

In some embodiments, the polymer composition is formed by reacting thepolybenzimidazole with the melamine-formaldehyde polymer such that atleast a portion of the polybenzimidazole is bonded to or cross-linkedwith the melamine-formaldehyde polymer. For example, thepolybenzimidazole and the melamine-formaldehyde polymer may react toform an IPN. The polybenzimidazole and melamine-formaldehyde polymer maybe combined and then heated to form the polymer composition. The polymercomposition may be formed into a variety of structures including films,membranes, fibers or bulky structures. For example, the polymercomposition may be formed into a free-standing film having a thicknessof between about 0.5 μm and about 100 μm and, more particularly, betweenabout 5 μm and about 30 μm. Structures formed from the polymercomposition, such as thin films, are substantially stable and may besubmerged in water for extended periods without crazing or cracking. Thepolymer composition enables free-standing films to be formed withoutadditives (e.g., phosphoric acid) that are used to form conventional PBIcompositions. Such thin films are, thus, useful as membranes for gasseparation processes and as coatings on substrates, such as metals,fibers or ceramics.

Polybenzimidazole is a heterocyclic polymer having the followingchemical structure:

where n is a number of repeats of the polybenzimidazole monomer. Forexample, n may be a number between about 10 and about 210. The molecularweight range for PBI is between about 3,000 grams/mole and about 65,000grams/mole (Daltons) and, more particularly, between about 10,000grams/mole and about 25,000 grams/mole. The polybenzimidazole includesimidazole functional groups, each having a nitrogen atom at the1-position and at the 3-position. During the reaction, thepolybenzimidazole may be modified by N-substitution (i.e., bonding themelamine-formaldehyde polymer to at least one of the nitrogen atoms ofat least one of the imidazole functional groups). The polybenzimidazolemay be, for example, CELAZOLE® polybenzimidazole (PBI), which may beobtained commercially from PBI Performance Products, Inc. (Charlotte,N.C.). Commercially available polybenzimidazoles (e.g., CELAZOLE®polybenzimidazole) may include additives, such as lithium chloride,sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid, forstabilization during storage. Thus, the polybenzimidazole used to formthe polymer composition may be substantially pure and free from suchadditives. Such a polybenzimidazole may be prepared by selectivepolymerization of 2,3,5,6-tetraminotoluene (TAT) with varioussubstituted and unsubstituted aromatic diacids, such as2,5-dihydroxyterephthalic acid (DHTA).

The polybenzimidazole may be dissolved in a polar, aprotic solvent toform a polybenzimidazole solution. For example, the polar, aproticsolvent may include at least one of N,N-dimethylacetamide (DMAc),dimethyl sulfoxide (DMSO), N,N-dimethylforamide (DMF) andN-methylpyrrolidinone (NMP). In one embodiment, the polar, aproticsolvent is DMAc. The polybenzimidazole solution may include betweenabout 0.5 percent by weight (wt %) and about 10 wt % ofpolybenzimidazole and, more particularly, about 2.5 wt %polybenzimidazole. Since polybenzimidazole is difficult to processwithout additives, the concentration of the polybenzimidazole in thepolybenzimidazole solution may be optimized to provide reliable filmformation. The polybenzimidazole may be stirred in the polar, aproticsolvent for a sufficient amount of time for the polybenzimidazole todissolve. Dissolving the polybenzimidazole in the polar, aprotic solventmay provide improved processability, as will be described.

The melamine-formaldehyde polymer may be selected from a number ofdifferent polymers having different organic side groups attached at amethyoxy group. By way of example and not limitation, the organic sidegroups may include an alkyl, such as methyl(CH₃), ethyl(C₂H₅),propyl(C₃H₅), isopropyl(C₃H₇), butyl(C₄H₉), pentyl(C₅H₁₁), hexyl(C₆H₁₃),and isomers thereof. For example, the melamine-formaldehyde polymer mayinclude at least one of methylated, poly(melamine-co-formaldehyde),poly(melamine-co-formaldehyde) butylated,poly(melamine-co-formaldehyde)isobutylated, andpoly(melamine-co-formaldehyde)methylated/butylated (55/45). The organicside groups may act as leaving groups during N-substitution, as will bedescribed.

The polybenzimidazole solution may be combined with themelamine-formaldehyde polymer to form a precursor. As a non-limitingexample, the melamine-formaldehyde polymer may be methylated,poly(melamine-co-formaldehyde) (CAS Number: 68002-20-0; M-PMF), whichhas the following chemical structure:

where R is independently selected from hydrogen (H) and an alkyl, suchas methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, and isomersthereof, and m is a number of repeats of thepoly(melamine-co-formaldehyde). For example, m may be a number betweenabout 1 and about 16. As a non-limiting example, the molecular weightrange for the methylated, poly(melamine-co-formaldehyde) may be betweenabout 390 grams/mole and about 6,500 grams/mole. For example, themethylated, poly(melamine-co-formaldehyde) may be obtained commerciallyfrom Sigma-Aldrich Biotechnology L.P. (Saint Louis, Mo.).

Before combining the melamine-formaldehyde polymer and thepolybenzimidazole solution, the melamine-formaldehyde polymer may bedissolved in an organic solvent, such as an ether or an alcohol, to forma melamine-formaldehyde polymer solution. In embodiments in which themelamine-formaldehyde polymer solution includes methylated,poly(melamine-co-formaldehyde), a concentration of methylated,poly(melamine-co-formaldehyde) in solution may be between about 1 wt %and about 95 wt %. For example, the melamine-formaldehyde polymer may bedissolved in butanol to form the melamine-formaldehyde polymer solution.The melamine-formaldehyde polymer solution may be combined with thepolybenzimidazole solution and may be subjected to mixing to form theprecursor including a polymer blend of melamine-formaldehyde polymer andpolybenzimidazole in solution.

The amount of melamine-formaldehyde polymer in the precursor may be lessthan about 80 wt % and, more particularly, between about 10 wt % andabout 50 wt %. For example, the precursor may be formed to include aratio of the polybenzimidazole:melamine-formaldehyde polymer of betweenabout 1:1 and about 20:1 and, more particularly, between about 2:1 toabout 5:1. In embodiments in which the precursor will be used to form abulky structure, such as a support rod, the amount ofmelamine-formaldehyde polymer in the precursor may be between about 1 wt% and about 80 wt %. In embodiments in which the precursor will be usedto form a thin film or coating, the amount of melamine-formaldehydepolymer in the precursor may be less that about 40 wt % and, moreparticularly, between about 10 wt % and about 20 wt %, to provide apolymer composition exhibiting a desired strength and resiliency.Increasing the amount of melamine-formaldehyde polymer in the precursorabove about 40 wt % may result in increased rigidity and brittleness inthe resulting polymer composition, which may be desirable inapplications in which the polymer composition will not be repeatedlysubjected to flexion. If the amount of the melamine-formaldehyde polymerin the precursor is greater than about 40 wt %, the precursor may stillform a clear solution and form free-standing films with varied physicalproperties.

The precursor may have a flowable consistency at room temperature or ata processing temperature that enables the precursor to be cast, forexample, into a mold or onto a substrate. The precursor may be cast, forexample, onto a substrate or into a mold. As a non-limiting example, athin layer of the precursor may be formed over a substrate (e.g., aglass slide). As another non-limiting example, the precursor may beheated, introduced into a mold and compressed to form a bulk article,such as a support rod.

To form the polymer composition, the solvents (i.e., the polar, aproticsolvent and the organic solvent) may be removed from the precursor andthe polybenzimidazole and the melamine-formaldehyde polymer may bereacted. The reaction between the polybenzimidazole and themelamine-formaldehyde polymer may be initiated by exposure to heat.While not wishing to be bound by any particular theory, it is believedthat a temperature greater than a boiling point of the solvent and/orthe leaving groups (i.e., the organic side groups of themelamine-formaldehyde polymer) may be used to initiate polymerization.For example, the polybenzimidazole and the melamine-formaldehyde polymermay be exposed to temperatures of less than or equal to 200° C. Inembodiments in which the melamine-formaldehyde polymer is methylated,poly(melamine-co-formaldehyde), the polybenzimidazole and themethylated, poly(melamine-co-formaldehyde) may be exposed to atemperature of between about 150° C. and about 250° C. to initiatepolymerization.

After initiation of polymerization, the intermediate composition mayexhibit an increased viscosity in comparison to the polybenzimidazolesolution. While not wishing to be bound by any particular theory, it isbelieved that such increased viscosity may occur as a result ofstabilization of the polybenzimidazole by the melamine-formaldehydepolymer and/or cross-linking of the polybenzimidazole with themelamine-formaldehyde polymer. After casting the precursor, the solventsmay be removed from the precursor by evaporation. For example, theprecursor may be exposed to a temperature sufficient to acceleratevaporization of the solvents from the precursor. In embodiments in whichthe polybenzimidazole is dissolved in DMAc and the melamine-formaldehydepolymer is dissolved in butanol, the solvents may be evaporated from theprecursor by exposing the precursor to a temperature of greater thanabout 150° C.

In embodiments in which the melamine-formaldehyde polymer is methylated,poly(melamine-co-formaldehyde), the reaction of the polybenzimidazolewith the methylated, poly(melamine-co-formaldehyde) may proceedaccording to the following reaction scheme:

where n is a number between about 5 and about 210, m is a number betweenabout 1 and about 16, each of x, y and z is a number between about 1 andabout 500 and R is independently selected from H and an alkyl, such asmethyl. During the reaction, the polybenzimidazole may be modified byN-substitution (i.e., bonding of the methylated,poly(melamine-co-formaldehyde) to at least one of the nitrogen atoms ofat least one of the imidazole functional groups of thepolybenzimidazole). The nitrogen atoms of the polybenzimidazole, thus,provide a reactive point for bonding of the melamine-formaldehydepolymer (e.g., the methylated, poly(melamine-co-formaldehyde). While notwishing to be bound by any particular theory, it is believed that thepolybenzimidazole reacts with alcohol and/or ether linkages of themelamine-formaldehyde polymer to produce a reaction product of thepolybenzimidazole and the melamine-formaldehyde polymer (i.e., thepolymer composition), and water and/or methanol as a byproduct.

The polybenzimidazole and the melamine-formaldehyde polymer may reactwith one another as previously described to form the reaction product ofthe polybenzimidazole and the melamine-formaldehyde polymer (i.e., thepolymer composition) that includes polybenzimidazole N-substituted with,or cross-linked by, the methylated, poly(melamine-co-formaldehyde). Thereaction between the polybenzimidazole and the melamine-formaldehydepolymer may be initiated by exposing the precursor to heat. Aspreviously described, the precursor may be exposed to a temperature ofless than or equal to 250° C. to initiate the reaction.

The polybenzimidazole and the methylated,poly(melamine-co-formaldehyde), respectively, may self-polymerize toform polymer segments including the polybenzimidazole (i.e.,polybenzimidazole segments) and polymer segments including themelamine-formaldehyde polymer (i.e., methylated,poly(melamine-co-formaldehyde) segments). The melamine-formaldehydepolymer may react with the imidazole nitrogen atoms of at least onepolybenzimidazole segment or the polybenzimidazole to form the polymercomposition. Crosslinking of the polybenzimidazole may occur when two ormore of the polybenzimidazole segments or polybenzimidazole react with asingle melamine-formaldehyde or melamine-formaldehyde polymer segment.For example, the polybenzimidazole and the melamine-formaldehyde polymermay polymerize to form an interpenetrating polymer network including twoor more interlaced, crosslinked segments including themelamine-formaldehyde polymer and the polybenzimidazole connected bycovalent bonds.

The resulting polymer composition may include some degree ofcrosslinking between polybenzimidazole and the melamine-formaldehydepolymer. In addition, the polymer composition may include themelamine-formaldehyde polymer segments and the polybenzimidazolesegments formed, respectively, by self-polymerization of themelamine-formaldehyde polymer and the polybenzimidazole. Themelamine-formaldehyde polymer and the polybenzimidazole may be attachedby stable bonds that include a methylene group spacer between amines. Anamount of time that the precursor may be exposed to the heat to form thepolymer composition may be determined based on the amount ofmelamine-formaldehyde polymer in the precursor. As the amount ofmelamine-formaldehyde polymer in the precursor increases, the amount oftime may be reduced. While not wishing to be bound by any particulartheory, an increased amount of the melamine-formaldehyde polymer in theprecursor may result in increased cross-linking of the polybenzimidazoleby the melamine-formaldehyde polymer and increased brittleness in theresulting polymer composition.

In some embodiments, a conventional spinning process (e.g.,dry-spinning, wet-spinning and dry-jet wet-spinning) may be used to formfibers of the polymer composition. As a non-limiting example, the fibersmay be spun from the polymer composition into a hot nitrogen atmosphereand steam stretched in tandem with spinning. For example, the fibers maybe formed by forming a precursor having the desired viscosity andevaporating the solvent from the precursor until it becomes viscousenough for fiber formation. The fibers of the polymer composition may beformed as the solvent is vaporized from the precursor. For example, suchfibers may be woven to form high temperature fabrics for used inprotective apparel, flight suits, foundry curtains, and aircraftfurnishings. Such fibers may also be used to form a hollow fibermembrane, which may be used, for example, in membrane separationprocesses for treating aqueous solutions at room temperature, includingwastewater treatment processes and biological membrane processes. As anon-limiting example, the hollow fiber membrane may be formed byfabricating a tubular membrane from the fibers of the polymercomposition and coating the tubular membrane with an active material.The active material may be formed from the polymer composition of thepresent disclosure, or another suitable polymer such as polysulfone,polyethersulfone, polyimide, polyetherimide, polyacrylonitrile,polyethylene terephthalate, polybutylene terephthalate, nylon,polyphenyl sulfite, polyethylene or polypropylene.

After the reaction, the polymer composition may be dried to removeremaining liquids (i.e., water and/or the solvents). For example, thepolymer composition may be exposed to a temperature of about 150° C. toevaporate remaining liquids therefrom. The polymer composition may betransparent or translucent and may have be clear, yellow to orange orlight brown to brown in color.

As described in the following examples, thin films formed from thepolymer composition may exhibit thermal stability (i.e., do notthermally degrade) at a temperature of up to 325° C., as well asmechanical resilience and resistance to moisture and water. It has beenobserved that such thin films can be exposed to water for very longperiods of time (e.g., about 2 weeks) without showing stress fracturing.

The following examples serve to explain embodiments of the presentinvention in more detail. These examples are not to be construed asbeing exhaustive or exclusive as to the scope of this invention.

EXAMPLES Example 1

Polybenzimidazole (10 grams) was pulverized into a fine powder andplaced into a 250 mL round-bottom flask. The round-bottom flask wasequipped with a water jacketed condenser, a gas inlet adapter andmagnetic stir bar to form a system. This system was placed under vacuumfor about 4 hours and then purged with nitrogen gas. Anhydrous DMAc(about 100 mL) was transferred to the round-bottom flask and the mixtureof the polybenzimidazole and the DMAc was heated to boiling withstirring for about 48 hours. The resulting solution of polybenzimidazolein DMAc was cooled to room temperature, gradually becoming more viscous.

A solution of polybenzimidazole in DMAc (10 ml; 0.0004 mol) wastransferred by syringe into an 8 dram scintillation vial. Then, 0.128 g(10:1 mass ratio) of 80 wt % of methylated,poly(melamine-co-formaldehyde) in butanol (average molecular weight ofabout 511 g/mol) was added directly into the polybenzimidazole solutionand mixed vigorously to form a precursor. The precursor included about10 wt % of the methylated, poly(melamine-co-formaldehyde). The precursorwas mixed until a substantially homogeneous state was reached. Theprecursor was mixed to obtain a substantially homogeneous solution. Theprecursor was cast onto a glass slide, and the solvent was evaporatedusing a temperature controlled hotplate. After evaporation of thesolvent, a film exhibiting transparency and having an orange-brown colorremained on the glass slide. The film was exposed to a temperature ofabout 150° C. for between about 24 hours to about 72 hours. Duringheating the polybenzimidazole and the methylated,poly(melamine-co-formaldehyde) may be at least partially cross-linked(e.g., between about 75% and about 95% cross-linked). The film wasexposed to a temperature of between about 200° C. and about 250° C. forbetween about 6 hours and about 24 hours. The resulting film had a lightbrown to brown color. The film was slowly cooled to room temperature,and was then lifted from the glass slide by immersion into distilledwater. The free-standing film was exposed to a temperature of about 150°C. to remove remaining water. After removal of the water, the film had athickness of between about 10 μm and about 20 μm.

Example 2

A solution of polybenzimidazole in DMAc (10 ml; 0.0004 mol) wastransferred by syringe into an 8 dram scintillation vial. Then, 0.250 g(5:1 mass ratio) of 80% by weight of methylated,poly(melamine-co-formaldehyde) in butanol is added directly into thepolybenzimidazole solution and mixed vigorously to form a precursor. Theprecursor was mixed to obtain a substantially homogeneous solution. Theprecursor included about 20 wt % of the methylated,poly(melamine-co-formaldehyde). The precursor was cast onto a glassslide, and the solvent was evaporated using a temperature controlledhotplate to reach a temperature to evaporate the solvent,dimethylacetamide. After evaporation of the solvent, a film exhibitingtransparency and having an orange-brown color remained on the glassslide. The film was exposed to a temperature of about 150° C. forbetween about 24 and about 72 hours. The film was exposed to atemperature of between about 200° C. and about 250° C. for between about6 hours and about 24 hours. The resulting film had a light brown tobrown color. The film was slowly cooled to room temperature, and waslifted from the glass plate by immersion in distilled water. Thefree-standing film was exposed to a temperature of about 150° C. toremove excess water.

Example 3

FIG. 1 is a Fourier transform infrared (FT-IR) spectrum showing acomparison of pure polybenzimidazole (PBI RESIN) with polymercompositions formed according to embodiments of the disclosed method.Compositions A, B and C were formed from precursors including 10 wt %,20 wt % and 40 wt % of the methylated, poly(melamine-co-formaldehyde),respectively, in solution with the polybenzimidazole. The spectrum showsseveral differences between the PBI RESIN and the polymer compositions.In particular, absorbencies between about 3600 cm⁻¹ and about 3300 cm⁻¹show that, as the concentration of the methylated,poly(melamine-co-formaldehyde) is increased in the polymer compositions(i.e., Compositions A, B and C), the methylated,poly(melamine-co-formaldehyde) dominates the polybenzimidazoleabsorbencies. The absorbencies between about 1300 cm⁻¹ and about 650cm⁻¹ show differences between the PBI RESIN and each of Compositions A,B and C. The IR spectral differences demonstrate the molecular changesof PBI and include dominate signals of the melamine formaldehyde. Theabsorbances for PBI are less intense in comparison to those for themelamine polymer, and dominate the spectrum at the higher melaminepercentages.

Example 4

Dynamic mechanical analysis (DMA) has provided additional insight intothe physical properties of the polymer compositions formed according tothe disclosed embodiments. Since it is difficult to obtain an accurateglass transition temperature (T_(g)) for polybenzimidazole usingdifferential scanning calorimetry (DSC), DMA was used to approximateglass transition temperatures for the polymer compositions. Purepolybenzimidazole has a glass transition temperature of about 425° C.and a storage modulus at about 5 GPa. DMA showed that a polymercomposition formed from a precursor that included about 40 wt % of themethylated, poly(melamine-co-formaldehyde) in solution with thepolybenzimidazole has a glass transition temperature (i.e., a tan deltamaximum) of about 350° C. While not wishing to be bound by anyparticular theory, it is believed that the glass transition temperaturefor the polymer composition is reduced in comparison to purepolybenzimidazole by incorporation of the methylated,poly(melamine-co-formaldehyde). The starting storage modulus of thepolymer composition was about 20 GPa, which is significantly increasedin comparison to that of the pure polybenzimidazole, suggesting that themethylated, poly(melamine-co-formaldehyde) and the polybenzimidazole arephysically linked together and act synergistically. Polymer compositionshaving a storage modulus of greater than or equal to 10 GPa are ofsignificant interest in the art of high performance polymers becausemost known polymers do not exhibit these types of strengths.

Example 5

A conventional gas permeability apparatus was used to perform acomparison of gas permeability in polymer films formed from a polyimide,polybenzimidazole, and a polymer composition including polybenzimidazoleand the methylated, poly(melamine-co-formaldehyde). Polyimides arecommonly used high performance polymers. For example, KAPTON® HNpolyimide film, which is available commercially from E. I. du Pont deNemours and Company (Wilmington, Del.), has been extensively used as ahigh performance polymer. Thus, the KAPTON® HN polyimide film was usedas a control for the high temperature gas permeation analysis. TheKAPTON® HN polyimide film had a thickness of about 12.5 μm.

A polybenzimidazole solution was formed by dissolving CELAZOLE® resin inDMAc. The polybenzimidazole solution was cast on a glass slide andsubjected to an annealing process to form the polybenzimidazole film.However, when water was used to lift the polybenzimidazole film from theglass slide, a shattered, fragmented polybenzimidazole film resulted.Thus, the polybenzimidazole film was formed by casting thepolybenzimidazole solution on a metal fit and subjecting the solution toan annealing process. The metal fit was obtained commercially from MottCorporation (Farmington, Conn.). The metal frit promoted adhesion andprevented fracture of the polybenzimidazole film. The polybenzimidazolemembrane had a thickness of about 20 μm.

The polymer film formed from the polymer composition including thepolybenzimidazole and the methylated, poly(melamine-co-formaldehyde)(“PBI/M-PMF film”) was formed from a precursor using methods similar tothose described in Examples 1 and 2. A ratio of thepolybenzimidazole:methylated, poly(melamine-co-formaldehyde) in theprecursor was about 3:1. After evaporating the solvent from theprecursor and annealing the precursor to form the PBI/M-PMF film, thePBI/M-PMF film was lifted from the glass slide using water. ThePBI/M-PMF film was substantially more rugged than the polybenzimidazolefilm formed in the absence of the metal frit and did not shatter orfracture during the lifting process. The PBI/M-PMF film had a thicknessof about 20 μm.

Characteristics of the KAPTON® HN polyimide film, the polybenzimidazolefilm and the PBI/M-PMF film as gas separation membranes were evaluatedand compared.

Gas permeability was determined for the polymer films in a pure gassystem and in a mixed gas system using a conventional time-lag method.In the pure gas system, each of the polymer films was separately exposedto hydrogen (H₂), methane (CH₄) and carbon dioxide (CO₂) to determinethe permeability of the polymer membranes to each of these gases. Bothsides of the polymer film being tested were evacuated to an equal vacuumpressure in a test cell. The test cell was then isolated and thepressure at time zero used as a baseline. A feed side of each of thepolymer films was separately exposed to hydrogen (H₂), methane (CH₄) orcarbon dioxide (CO₂), and pressure buildup on a permeate side of thepolymer film opposite the feed side was recorded as a function of time.The permeability data was obtained at a temperature of about 30° C. anda feed gas pressure of about 30 psi. Table 1 summarizes selectivity andpermeability determined directly from the pure gas system.

TABLE 1 Selectivity α Permeability (barrers)^(a) H₂/ Polymer Film H₂ N₂O₂ CH₄ CO₂ H₂/CO₂ CO₂/CH₄ CH₄ KAPTON ® HN 1.56 0.01 0.075 0.008 0.2975.3 37.1  195.0 polyimide film PBI film 4.6 0.12 0.29 0.13 0.85 5.4 6.5 35.4 PBI/M-PMF 2.07 N/A N/A 0.002^(b) 0.08 25.8 40.4 1034^(b) film^(a)Permeabilities measured in barrers: [(10⁻¹⁰)((cm³ (STP) × cm)/(cm² ×sec × cmHg))] ^(b)Data are at instrument detection limits

In the mixed gas system, permeability was determined analytically by gaschromatography to detect gas concentrations. Each of the polymer filmswas exposed to gas pairs including hydrogen and carbon dioxide (H₂/CO₂),carbon dioxide and methane (CO₂/CH₄) and hydrogen and methane (H₂/CH₄),to determine permeability of the polymer films to each of the gas pairs.Helium was used as a carrier gas to sweep the mixed gas pairs from asurface of the polymer films. The mixed gas system may reachtemperatures of up to about 400° C. Thus, the mixed gas system was usedto perform gas permeation analysis at high temperatures (i.e., attemperatures of greater than 70° C.).

The polymer films were separately exposed to each of the gas pairs usinga conventional mixed gas permeation apparatus at a temperature of about250° C. for about 20 hours. Table 2 summarizes selectivity andpermeability determined directly from the mixed gas system at 250° C.

TABLE 2 Permeability (barrers)^(a) Selectivity α Polymer Film H₂ CH₄ CO₂H₂/CO₂ CO₂/CH₄ H₂/CH₄ KAPTON ® HN 37.5 4.1 11.8 3.2 2.8 9.2 polyimidefilm PBI film 33.1 1.69 4.7 7.1 2.8 2.8 PBI/M-PMF film 56.8 0.69 4.3912.9 6.4 82.3 ^(a)Permeabilities measured in barrers: [(10−10)((cm³(STP) × cm)/(cm² × sec × cmHg))]

As shown in Tables 1 and 2, in comparison to the KAPTON® HN polyimidefilm and the polybenzimidazole film, the PBI/M-PMF film exhibitedsubstantially increased gas selectivity (α) for each of hydrogen,methane and carbon dioxide. As shown in Table 2, the PBI/M-PMF filmprovided a substantial increase in the gas selectivity in comparison tothe polybenzimidazole film at 250° C. The gas selectivity of thePBI/M-PMF film is substantially greater than many conventional highperformance polymers (e.g., the KAPTON® HN polyimide). The PBI/M-PMFfilm exhibited increased selectivity for H₂ over CO₂ (α=12.9), CO₂ overCH₄ (α=6.4) and H₂ over CH₄ (α=82.3) at 250° C. in comparison to theKAPTON® HN polymer film and the PBI polymer film. In addition, thePBI/M-PMF film retained its physical properties (e.g., resistance toorganic solvents and chemical and thermal stability) during exposure tothe gases.

Table 3 provides a summary of the gas permeability of the PBI/M-PMF filmover time. As shown in Table 3, hydrogen permeability and H₂/CO₂separation factor of the PBI/M-PMF film significantly increased overtime, whereas carbon dioxide permeability, methane permeability andCO₂/CH₂ separation factor remained substantially constant. Overall, thedata shown in Table 3 demonstrated that the PBI/M-PMF film will be ableto perform as a gas separation membrane for long periods of time (i.e.,at least 20 hours) without losing gas selectivity.

TABLE 3 Time Permeability (Barrers) Separation Factor (hrs) H₂ CO₂ CH₄CO₂/CH₄ H₂/CO₂ 2 34.77 4.36 0.72 6.1 8.0 4 36.91 4.03 0.6 6.7 9.2 6 38.43.85 0.54 7.1 10.0 8 54.41 3.92 0.56 7.0 13.9 10 43.31 3.97 0.67 5.910.9 12 49.15 4.52 0.62 7.3 10.9 14 51.91 4.67 0.8 5.8 11.1 16 51.834.05 0.61 6.6 12.8 18 54.33 4.06 0.63 6.4 13.4 20 56.79 4.39 0.69 6.412.9

While the invention is susceptible to various modifications andimplementation in alternative forms, specific embodiments have beenshown by way of non-limiting example in the drawing and have beendescribed in detail herein. It should be understood that the inventionis not intended to be limited to the particular forms disclosed. Rather,the invention includes all modifications, equivalents, and alternativesfalling within the scope of the following appended claims and theirlegal equivalents.

What is claimed is:
 1. A polymer composition, comprising: at least onepolybenzimidazole segment; and at least one melamine-formaldehydepolymer segment at least partially intertwined with the at least onepolybenzimidazole segment.
 2. The polymer composition of claim 1,wherein the at least one melamine-formaldehyde polymer segment comprisesat least one methylated, poly(melamine-co-formaldehyde) segment.
 3. Thepolymer composition of claim 1, wherein a concentration of the at leastone polybenzimidazole segment within the polymer composition is greaterthan a concentration of the at least one melamine-formaldehyde polymersegment within the polymer composition.
 4. The polymer composition ofclaim 1, wherein the at least one melamine-formaldehyde polymer segmentis bonded to at least a portion of nitrogen atoms of the at least onepolybenzimidazole segment.
 5. The polymer composition of claim 1,wherein the polymer composition is substantially free of acidiccompounds and lithium salts.
 6. A polymer film, comprising: a film of apolymer composition, the polymer composition comprising: at least onepolybenzimidazole segment; and at least one melamine-formaldehydesegment at least partially intertwined with the at least onepolybenzimidazole segment.
 7. The polymer film of claim 6, wherein amelamine-formaldehyde polymer of the at least one melamine-formaldehydesegment comprises methylated, poly(melamine-co-formaldehyde).
 8. Thepolymer film of claim 6, wherein the film of the polymer composition hasa thickness of between about 0.5 μm and about 100 μm.
 9. The polymerfilm of claim 6, wherein the film of the polymer composition has athickness of between about 5 μm and about 30 μm.
 10. The polymer film ofclaim 6, wherein the film of the polymer composition is substantiallyflexible.
 11. The polymer film of claim 6, wherein the at least onemelamine-formaldehyde segment is reacted with at least a portion ofnitrogen atoms of the at least one polybenzimidazole segment.
 12. Thepolymer film of claim 6, wherein the film of the polymer compositionexhibits substantially increased hydrogen permeability andhydrogen/carbon dioxide separation over time.
 13. A method for forming apolymer composition, comprising: combining a melamine-formaldehydepolymer and polybenzimidazole to form a mixture; and heating the mixtureto form a polymer composition, the melamine-formaldehyde polymer atleast partially intertwined with the polybenzimidazole.
 14. The methodof claim 13, wherein combining a melamine-formaldehyde polymer andpolybenzimidazole to form a mixture comprises forming the mixturecomprising less than or equal to about 40% by weight of themelamine-formaldehyde polymer.
 15. The method of claim 13, whereincombining a melamine-formaldehyde polymer and polybenzimidazole to forma mixture comprises forming the mixture comprising between about 10% byweight and about 20% by weight of the melamine-formaldehyde polymer. 16.The method of claim 13, wherein combining a melamine-formaldehydepolymer and polybenzimidazole to form a mixture comprises forming themixture comprising a ratio of between about 1:1 and about 20:1 of themelamine-formaldehyde polymer to the polybenzimidazole.
 17. The methodof claim 13, wherein combining a melamine-formaldehyde polymer andpolybenzimidazole to form a mixture comprises forming the mixturecomprising a ratio of between about 2:1 and about 5:1 of themelamine-formaldehyde polymer to the polybenzimidazole.
 18. The methodof claim 13, wherein combining a melamine-formaldehyde polymer andpolybenzimidazole to form a mixture comprising forming the mixtureconsisting of methylated, poly(melamine-co-formaldehyde), thepolybenzimidazole, and at least one solvent.
 19. The method of claim 13,wherein heating the mixture to form a polymer composition comprisesreacting the melamine-formaldehyde polymer with the polybenzimidazole toform the polymer composition.
 20. The method of claim 19, whereinreacting the melamine-formaldehyde polymer with the polybenzimidazole toform the polymer composition comprises bonding methylated,poly(melamine-co-formaldehyde) to at least a portion of nitrogen atomsof the polybenzimidazole.
 21. The method of claim 13, wherein combininga melamine-formaldehyde polymer and polybenzimidazole to form a mixturecomprises: dissolving the polybenzimidazole into a polar, aproticsolvent to form a polybenzimidazole solution; dissolving methylated,poly(melamine-co-formaldehyde) into an organic solvent to form amethylated, poly(melamine-co-formaldehyde) solution; and combining thepolybenzimidazole solution and the methylated,poly(melamine-co-formaldehyde) solution to form the mixture.
 22. Themethod of claim 21, further comprising heating the mixture to atemperature of between about 150° C. and about 250° C. to remove thepolar, aprotic solvent and the organic solvent.